1. Field of the Invention
The present invention relates to an organic EL (Electro Luminescence) element drive circuit and an organic EL display device and, in particular, the present invention relates to an organic EL element drive circuit capable of preventing erroneous light emission of matrix-arranged organic EL elements and reducing power consumption thereof and an improvement of an organic EL display device.
2. Description of the Related Art
Since an organic EL display device can perform a high luminance display due to spontaneous light emission, the organic EL display device is suitable for use in a display device whose display screen size is small and is expected as the next generation display device to be mounted on such as a portable telephone set, a DVD player or a PDA (personal digital assistance), etc. A known problem of the organic EL display device is that variation of luminance becomes considerable when a voltage drive is applied to the organic EL display device as in a liquid crystal display device and the drive control becomes difficult due to the difference in sensitivity between R (red), G (green) and B (blue).
In view of this problem, an organic EL display device using a current driver is proposed recently. For example, in JP H10-112391A, a technique for solving the problem of luminance variation by employing the current drive is disclosed.
In a recent organic EL display panel of an organic EL display device for use in a portable telephone set, the number of terminal pins of column lines is 396 (132×3) and the number of terminal pins of row lines is 162. These numbers of the terminal pins are still increasing.
An output stage of each of current drive circuits of such organic EL display panel includes a drive circuit of a power source corresponding to each terminal pin such as, for example, a current mirror output circuit regardless of the type of driving, that is, the active matrix type or the simple matrix type. In, for example, U.S. application Ser. No. 10,102,671, which corresponds to Japanese Application JP 2002-82662 claiming domestic priorities of Japanese Application JP2001-86967 and Japanese Application JP2001-396219, a drive stage includes a parallel driven current mirror circuit (reference current distribution circuit) having output side transistors the number of which corresponds to the number of terminal pins and drives the output circuit by generating a corresponding number of mirror currents on the basis of a reference current supplied from a reference current generator circuit provided precedent to an input of the drive stage and distributing these mirror currents to the respective terminal pins. Alternatively, the mirror currents distributed to the terminal pins are amplified by k times (k is an integer equal to or larger than 2) and drive the output circuits. The k-time amplifier circuit is disclosed in Japanese Application JP2002-33719 assigned to the assignee of this application, in which a D/A converter circuit is provided for each terminal pin. In the k-time amplifier circuit, the D/A converter circuits corresponding to the respective column side terminal pins receive display data and column side drive currents for the respective terminal pins are generated simultaneously by A/D converting the column data.
It is general, in the organic EL display device, that one of the column side (anode side) lines becomes the current discharge side and the row side (cathode scan side) lines becomes the current sink side. Drive currents from the column side current drive circuits are supplied to the anode side of the organic EL elements correspondingly to the row side scan. The cathode side of the organic EL elements is grounded through CMOS push-pull circuits to sink the drive currents. Since the organic EL element is a capacitive element, a portion of the drive current is accumulated in the organic EL element as electric charge. Therefore, in the display device having a matrix-arranged organic EL elements, charges may flow from the organic EL elements arranged in the peripheral portion of the panel, which are not to be scanned, into the organic EL element, which is to be scanned. Consequently, there is a problem that the organic EL elements, which are not scanned, emit light and/or the luminance of the driven organic EL elements varies, resulting in erroneous light emission.
FIG. 6 schematically shows an organic EL display panel of a conventional organic EL display device. The conventional organic EL display panel 1 includes matrix-arranged organic EL elements 4, column side current drive circuits 2 and row side drive circuits 3. In FIG. 6, the organic EL elements 4 are shown as capacitors and a CMOS push-pull circuit of the drive circuit 3 is shown as a pair of series connected switches, for convenience.
In the organic EL display panel 1, in order to improve the luminance of the organic EL elements 4 and to prevent the luminance thereof from being varied, the organic EL elements 4 are preliminarily charged for a constant time, which is determined by the junction capacitances thereof. Therefore, switch circuits SW each provided between the column side current drive circuit 2 and the ground line are made ON for a constant time before the drive is started, to discharge electric charges of the organic EL elements 4 to thereby reset the organic EL elements. The resetting of the organic EL elements is performed by making the switch circuits SW ON for an initial constant time for which a row side line of the row side drive circuit 3, which is to be scanned, becomes low (L) level to ground column lines (anode side lines) X1, X2, X3, . . . connected to outputs of the current drive circuits 2. Thus, residual charge of the organic EL elements 4 are discharged and, thereafter, the output currents of the column side current drive circuits 2 are supplied to the organic EL elements 4. In the row side drive circuits 3, the organic EL elements 4, which are to be not scanned, are reverse-biased. Otherwise, the drive current flows in the organic EL element 4, which is to be scanned, also flows into other organic EL elements arranged around the organic EL element 4, causing the erroneous light emission. Therefore, the row lines (cathode side lines) Y1, Y2, Y3, . . . , which are to be not scanned, are fixed to High (H) level.
There is a recent tendency that the number of the drive pins is increased concomitantly with the request of higher resolution. When the number of the drive pins is increased, the drive frequency tends to become higher and the power consumption tends to increase. However, when, in order to prevent the erroneous light emission, the organic EL elements other than that to be scanned are reverse-biased on the row side, charge for the reverse-biasing is accumulated in the organic EL elements in a direction opposite to the drive direction. Therefore, when a certain row line becomes one to be scanned, a large transient current flows to drive the row line while canceling out the charge stored therein in the reverse direction. As a result, the increase of the power consumption due to current required to store the charge for the reverse-biasing and the drive current due to the transient current become not negligible when the number of the drive pins is increased.